Phosphorylation of ribosomal protein S6 in HeLa cells occurs maximally in response to fresh medium or insulin and variably in response to cyclic nucleotides. The possibility that the insulin response and the cyclic nucleotide response represent two different, and possibly antagonistic, metabolic pathways is under investigation. The function of S6 phosphorylation is still unknown, but we have recently found that the transit time for polypeptide synthesis in vivo is inversely correlated with the extent of S6 phosphorylation. An attempt will be made to test for a causal relationship between S6 phosphorylation and the elongation rate in a cell-free system.